the.com/blackguard

means A scoundrel or thoroughly unscrupulous person — one whose bad behavior is dressed up with smooth manners or low cunning.

from First recorded in 1500s England, originally referring to the lowest menial servants — kitchen boys, scullions, and camp followers who tended pots, boots, and grime, sometimes literally marching behind a great house's baggage train. Because this rabble was associated with dirt, idleness, and petty crime, 'blackguard' slid from 'grubby servant' to 'scoundrel' by the 1700s. The exact link between the servant corps and the guard sense is debated, but the class contempt behind the word is well documented.